DS10 had a few friends from school over, meanwhile DS8 was out on a playdate of his own. When DS8 returned and DS10's friends were leaving, DS8 found that several of his favourite pokemon cards were missing from his binder, as well as some unopened booster packs he'd bought with his own money and was saving for later. He was distraught.
One of the kids, Child A, has had one playdate with us before, where some pokemon cards mysteriously disappeared from the kids' binders and then Child A 'found' them down the back of the sofa, beds, etc. We weren't overly impressed by this behaviour, but as they all turned up there was no harm done. However second time around it was clearly him again.
We searched around everywhere, and asked Child A to see if he could help find them. He miraculously 'found' the cards missing from DS8's binder but the unopened packs have not turned up. His mum was very apologetic and asked him to check if they were in his pockets, and he said they weren't. It was all super awkward. I didn't feel like we could accuse a 10yo of stealing with no proof, but he clearly has.
To be clear, my DS8 was out of the house and the cards were all in his bedroom, so it's not a case of a trade gone wrong or anything like that.
WWYD now, if anything? Other than get my poor DS8 some new cards and not have Child A round to our house again!